The present invention relates to a mold for producing molded parts from inorganic filler-containing crosslinkable liquid fluoroelastomer compositions, and to a method for producing molded fluoroelastomer parts using this mold. More particularly, it relates to a mold useful for molding fluoroelastomer parts in a liquid injection molding system (LIMS), and to a method for producing molded fluoroelastomer parts by LIMS molding with this mold.
Conventional vinylidene fluoride-type fluoroelastomers are endowed with many excellent properties, including heat resistance, chemical resistance and mechanical strength, and thus are used in a broad range of industrial applications, most particularly in the automotive and machine industries.
However, these prior-art fluoroelastomers have an inadequate resistance to chemicals. That is, they swell easily in polar solvents such as ketones, lower alcohols, carbonyls and organic acids, and they deteriorate in amine-containing chemicals, resulting in a drastic decline in elastomer strength and elongation. The low-temperature properties of such fluoroelastomers also leave something to be desired. For example, at temperatures below xe2x88x9220xc2x0 C., they lose their rubber-like elasticity, making them useless as seals. This has generally restricted their use in cold-weather climates.
One approach that has been taken to overcome these drawbacks is the use of curable fluorinated compositions composed primarily of perfluorocarbon compounds and fluorinated organohydropolysiloxanes.
Because perfluorocarbon compounds have a low degree of polymerization and are liquid, the above compositions are also liquid, making them well-suited to a formed-in-place gasket (FIPG) process or LIMS molding.
Moreover, because such compositions have excellent characteristics such as oil resistance, chemical resistance, low temperature properties and heat resistance, they are often used as seals in automobiles and aircraft. The needs that exist in such applications for improved strength and compression set, and for even better oil resistance and chemical resistance, have made selection of the reinforcing filler a topic of considerable importance.
To satisfy such needs, the common practice is to blend several different fillers into the composition so as to take advantage of the distinctive particle size and surface area characteristics of the different fillers.
However, some fillers, when present in a composition that is made to flow rapidly, will exhibit abrasive effects upon striking mold surfaces. Depending on the shape of the molded article, such fillers may give rise to wear in the mold, particularly in the gate portion of the mold.
O-rings and other types of seals need to be precisely molded to increase their airtightness. Because there are often very strict limits on defects such on flash and gate marks, it is necessary to use a mold construction in which areas of material flow such as gates and the cut-off are finished to very small (thin) dimensions. When the composition passes through these thin areas, depending on the type of filler, the mold may be subjected to abrasion for the reason mentioned above.
Filler that abrades the mold in such cases has an average particle size of at least 0.1 xcexcm. Relatively little mold wear occurs at smaller particle sizes.
LIMS molding is particularly suitable for mass production, in part because it is readily amenable to shortening or automation of the molding cycle. However, the molds used for this purpose are of complex construction and thus expensive compared with conventional two-piece rubber molds. Hence, mold wear is a serious problem. Although mold construction and surface treatment are of paramount importance for holding down production costs and maintaining consistent quality, an optimal mold construction and optimal surface treatment for the LIMS molding of liquid fluoroelastomer compositions have yet to be established. For this reason, it has been difficult to obtain finished products of consistent quality.
It is therefore an object of the invention to provide a mold for producing molded fluoroelastomer parts which prevents to the greatest possible extent wear of the mold gate by inorganic fillers, and is particularly useful in LIMS molding. Another object of the invention is to provide a method for producing molded fluoroelastomer parts using this mold.
We have found that by surface-treating with diamond-like carbon at least the gate portion of a mold for molding inorganic filler-containing crosslinkable liquid fluoroelastomer compositions and making the cross-sectional area of the gate opening on the side of the mold cavity (i.e., the surface area of the gate portion where it opens out into the cavity) at least 0.075 mm2, wear of the mold gate by the inorganic filler can be minimized, thereby increasing the useful life of the mold. In addition, the mold release properties are improved, resulting in molded fluoroelastomer parts of consistent quality and lower cost. We have additionally found that the amount of flash can be reduced by holding the thickness (depth) of the gate to not more than 0.2 mm.
Accordingly, the invention provides a mold for producing molded fluoroelastomer parts, the mold comprising a mold cavity, a gate through which an inorganic filler-containing crosslinkable liquid fluoroelastomer composition is injected into the mold cavity, a runner and a sprue through which the gate communicates with an injection molding machine. The gate is surface treated with diamond-like carbon and has a cross-sectional area where it opens out into the mold cavity of at least 0.075 mm2.
The invention additionally provides a method for producing molded fluoroelastomer parts, wherein an inorganic filler-containing crosslinkable liquid fluoroelastomer composition is injected through the gate and into the cavity of the above mold, and thereby molded.